UC3875LG-S08-R datasheet(6/11 Pages) UTC

6 / 11 page

UC3875L

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 11

www.unisonic.com.tw

QW-R103-125 .A



OPERATION DESCRIPTION

The UTC UC3875L devices integrate many useful designs into one controller for low-power switch-mode power

supplies. The following descriptions highlight some of the features of the UTC UC3875L series.

Start-up Current

The start-up current is only 5μA. Low start-up current allows a start-up resistor with a high resistance and a

low-wattage to supply the start-up power for the controller. For AC/DC adaptor with universal input range design, a

2.5~3MΩ, 1/8W startup resistor could be used together with a VDD capacitor to provide a fast startup and low power

dissipation solution.

Power-Saving Mode Operation

The proprietary Power-Saving Mode function provides linearly decreasing the switching frequency under light-load

conditions for higher efficiency. The feedback voltage, which is sampled from the voltage feedback loop, is taken as

the reference. Once the feedback voltage dropped below the threshold voltage, the switching frequency starts to

decrease. This Power-Saving Mode function dramatically reduces power consumption under light-load conditions.

The 22KHz minimum frequency control also eliminates the audio noise at any loading conditions.

At zero load condition, the magnitude of power loss is in proportion to the number of switching events within a

fixed period of time. Reducing switching events leads to the reduction on the power loss and thus conserves the

energy. The UTC UC3875L enter burst mode at standby condition to minimize the switching loss and reduces the

standby power consumption. Power supplies using the UTC UC3875L can easily meet even the strictest regulations

regarding standby power consumption.

Switch Frequency Set

The maximum switch frequency is set to 65KHz. Switch frequency is modulated by output power POUT during IC

operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from

switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the snubber circuit. The

magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower

switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light

load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between fSW

and POUT/POUT (MAX) as followed Fig.1.

1%~100% Load @ 115 & 230 Vac

POUT/POUT(max)

30%

19%

100%

115V

230V

Fig.1 The relation curve between fSW and relative output power POUT/ POUT (MAX)

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